1. Field of the Invention
The present invention relates to a slippage preventing apparatus for a vehicle with an automatic transmission.
2. Description of Related Art
Prevention of a large degree of slippage of the driven wheels on pavement permits an effective provision of an impellent force with the vehicle and is effective from a standpoint of safety in order to prevent the spinning of the vehicle. It is known that an increase in an amount of slippage of the driven wheel may be prevented by decreasing a torque to be applied to the driven wheels, which may cause slippage.
Japanese Patent Publication (laid-open) Nos. 16,948/1983 and 56,662/1985 disclose a slip control in which a torque to the driven wheels is decreased by applying a braking force to the driven wheels through the brake and by decreasing a torque output from the engine. More specifically, Japanese Patent Publication (laid-open) No. 16,948/1983 is directed to such a slip control as solely braking the driven wheels, on the one hand, when a degree of slippage is smaller, and as additionally decreasing the torque output from the engine, on the other hand, when a degree of slippage becomes larger. Japanese Patent Publication (laid-open) No. 56,662/1985 is directed to a slip control technology such that, when one of the left-hand and right-hand driven wheels slips to a larger degree than the other, only the one driven wheel is braked and, when the left-hand and right-hand driven wheels both slip to a greater degree, an output torque from the engine is reduced in addition to the braking against the driven wheels.
An increasing number of automobiles are equipped with an automatic transmission of the multispeed stage type that is designed to transmit an output of the engine to the driven wheels. Such an automatic transmission can automatically shift speed stages on the basis of shift characteristics which have been set in advance, for example, those set using a vehicle speed and a degree of throttle opening as parameters.
A vehicle equipped with such an automatic transmission may be constructed such that the speed stages are shifted upon the slip control. More specifically, in instances where the output of the engine is adjusted during operation of the slip control, the speed stages may be likely to be shifted as a throttle opening associated varies with the slip control if a factor such as, for example, a throttle opening, related to the engine output would be used as a parameter for shift characteristics. And the speed stages may be likely to be further shifted by the slip control against the driven wheels by means of an alteration in torques to be applied to the driven wheels involved in the shift of speed stages. Thus, for a vehicle with an automatic transmission, it may be likely to cause the upshifting and downshifting to be frequently repeated by means of an interaction between the slip control and the shift of the speed stages.